1. Technical Field
The present teaching relates to method and system for analog circuits. More specifically, the present teaching relates to method and system for step-up converters and systems incorporating the same.
2. Discussion of Technical Background
Step-up DC/DC converters are frequently used to boost a DC input voltage to a higher voltage. A common example is to boost a voltage from a single 1.5 VDC alkaline cell up to a regulated 3.3 VDC to power, e.g., analog or digital circuitry in a portable device. Conventionally, step-up converters can operate from input voltages as low as 1V, allowing them to be powered from a single cell. However, there are applications that must operate from an input voltage significantly less than 0.5V. Examples include applications where battery power is not practical, either due to an inhospitable environment or a remote location where having periodical access to replace batteries is impractical. In those situations, although alternate forms of energy may be an option to power the electronics, such as photovoltaic (PV) cells, thermopiles, and Peltier cells (also called thermo-electric coolers), these alternative energy sources produce an output voltage well below 1V, and in some cases just a few hundred millivolts or less.
Such low input voltages pose a problem for conventional DC/DC converters because they can not start or operate at an input voltage of a few hundred millivolts or less. One reason for that is that such a low input voltage is simply not high enough to forward bias the emitter-base junction of a transistor, or satisfy the threshold voltage of a typical MOSFET, making it impossible to power the converter.
Although a higher voltage may be achieved by putting multiple devices in series, such a solution increases size and cost. Another solution, which is well documented, is to use a depletion-mode transistor, such as a depletion-mode, N-channel JFET, and a step-up transformer with a high primary to secondary turns ratio. Since a depletion-mode device conducts current with no bias voltage applied to its gate, a free-running oscillator can be constructed, using the transformer to provide enough gain to oscillate and step-up the input voltage. Such designs can operate from an input voltage of 50 mV or less, generating an output voltage of several volts or more when a proper transformer turns ratio is provided.
FIG. 1(a) shows such a simplified implementation with a turns ratio of 1:100. In this prior art solution, the transformer T1 is connected to a power source 105 and produces an output voltage (SEC) at 140 to be sent to a rectifier. The secondary winding of transformer T1 (115 and 120) provides a sinusoidal output which is used to drive a depletion-mode JFET Q1 (125) on and off. A coupling capacitor 130 provides DC isolation from the secondary winding to the gate of 125 because the gate-source junction of Q1 125 clamps the positive peak voltage to a diode drop above ground. A high value resistor 135 connecting the gate of transistor 125 to ground provides a DC ground reference. The voltage on the secondary winding can then be rectified to produce a boosted DC output voltage. Typical waveforms observed in circuit 100 are shown in FIG. 1(b), in which waveform 150 represents the voltage observed at the drain terminal of transistor 125, waveform 160 represents the current flowing through the drain terminal of transistor 125, and waveform 170 represents the voltage 140 at SEC in FIG. 1(a).
For energy sources whose voltage polarity remains constant, the approach described in FIG. 1(a) works well. However, in some applications, the polarity of the input voltage may be unknown, or may change with time. For example, this situation will occur when a Peltier cell is used as the energy source. As commonly known, a Peltier cell generates a DC voltage based on the so-called “Seebeck effect” when a temperature differential is imposed across the cell. Due to the fact that the polarity of the output voltage of the Peltier cell depends on the “polarity” of the temperature differential across it, the polarity of the input voltage to the step-up converter powered by a Peltier dynamically changes. That is, in some applications, the “hot” and “cold” sides of the cell may switch depending on ambient conditions. In this case, a step-up converter using a Peltier cell needs to operate with either polarity input voltage. None of the existing techniques is capable of operating under such conditions.
The requirements to be “polarity independent” and the ability to operate from a very low input voltage of either polarity pose a major challenge to the prior art. All existing step-up converters, including the ones that can work with low input voltages, cannot start or operate if the wrong DC polarity is applied to their inputs.